Just give him a bill of sale and sign the title over to him. There are no “tax forms” for you to fill out on this.
If you used the car for business purposes then you might need to fill out a form 4797 on your tax return for the sale, but if all you used it for was just normal driving around, to work, grocery shopping, vacations, etc, then there’s no tax forms to fill out.
There is no Federal income tax forms. However, there are state sales tax forms (at least in Illinois). If/when you transfer the title, a Currency Exchange (or other service bureau) should supply you with forms. The amount is based on the sale price.
If you sold it for more than you paid for it, however unlikely, you would report the gain on Sch D. Losses on sale of personal use assets are not deductible